1. Field of the Invention
The present invention relates to a combined cathode and diaphragm unit for electrolytic cells, and more particularly the present invention relates to such units for electrolytic cells utilized for the electrolysis of sodium chloride.
2. Prior Art
The making of diaphragms for brine-electrolysis cells from asbestos has been widely practiced throughout the world for many decades. Those skilled in the art are familiar with the techniques involved, wherein asbestos is formed in a slurry and drawn through the cathode screen in a vacuum box to provide a diaphragm wherein the asbestos fibers are entwined with the screen and with each other. In this way, a unit is obtained that will remain intact during operation in spite of vigorous hydrogen generation at the cathode screen. One of the disadvantages of this construction is that the physical intimacy and/or the interaction between the asbestos and cathode screen interferes somewhat with the generation of hydrogen at the cathode and tends to increase electrical resistance due to bubble retention. Another disadvantage is the use of asbestos, the use of which is becoming increasingly regarded as a health hazard.
However, the use of substitutes for asbestos is a relatively new technology, and improvements in the technology of such substitutes is necessary to render the new technology practical. The present invention relates to one of such improvements. One satisfactory substitute which has been found and is known to the prior art is the use of relatively inert synthetic plastic material which may be formed into small fibers and deposited by known techniques to provide fibrous diaphragms. An example of such a diaphragm is shown in U.S. Pat. No. 4,036,729. Other improvements have been made in the use of these synthetic fibers to make satisfactory diaphragms, involving the use of fluorinated hydrocarbon resins, heat treatments, and the like in order to render the diaphragms more satisfactory. In addition, these synthetic fibers are hydrophobic and have presented difficulties not present with hydrophilic asbestos fibers. Accordingly, it is also known to utilize surfactants to render the fiber diaphragms more wettable.
As noted above, U.S. Pat. No. 4,049,841 discloses a cathode having special surface characteristics which may be combined with asbestos diaphragms and provide an improvement in lowering the hydrogen overvoltage of the cathode. Similarly, U.S. Pat. Nos. 3,945,907 and 3,974,058 show special cathodes having a low hydrogen overvoltage. However, it is believed that the advantages achieved by the combined cathode and diaphragm unit of this invention are not suggested by any of these prior art patents.